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Hydraulic cylinders are divided into single-acting cylinders and double-acting cylinders. The former means that the movement of the hydraulic cylinder in one direction is carried out by hydraulic pressure, while the movement in the other direction is carried out by external forces such as self-weight or springs. The movement in both directions of the double-acting cylinder is carried out under hydraulic pressure.

Due to the difference between the cylinder bore and rod diameter of the hydraulic cylinder, the output power in the two directions includes thrust and pull, and the movement speed in the two directions is different, and we often need it to be the same, or vice versa. In order to achieve this purpose, the following three basic working forms of hydraulic cylinders have appeared.

It should be pointed out that the research basis of these three types of hydraulic cylinders is the following two important formulas.

F=PS and v=Q/S.

Since it is not so convenient to extend or retract by changing P or Q, the following three structural forms are adopted.

Double-acting hydraulic cylinder with the basic structure

The characteristic of this hydraulic cylinder is that the piston and the rod body are a whole, the cross-sectional area of the piston in the left chamber is S1, the cross-sectional area of the piston in the right chamber is S2, and the cross-sectional area of the rod body is S3. The relationship between the three is S1=S2+S3.

Basic Hydraulic Cylinder

Push force, pull force, extension and retraction speed have the following relationship

Condition: P and Q remain unchanged.

F push=PS1, F pull=PS2=P (S1-S3), V stretch=Q/S1, V shrink=Q/S2=Q/(S1-S3),

Since S1>S2, F pushes>F pulls, and V stretches<V shrinks.

This kind of hydraulic cylinder has a simple structure and is mostly used in occasions that require high thrust, low tension, slow movement speed, and fast return, that is, heavy load out and no load back.

Floating hydraulic cylinder

The characteristic of this hydraulic cylinder is that the piston and the rod body are not integral, and can move on the rod body. The cross-sectional area of the piston is S2, and the cross-sectional area of the rod body is S3.

Floating hydraulic cylinder

Push force, pull force, extension and retraction speed have the following relationship

Condition: P and Q remain unchanged.

F push=PS3, F pull=PS2, V stretch=Q/S3, V shrink=Q/S2.

The relationship between S2 and S3 can be >, <, =, so F pushing can be >, <, = F pulling, V stretching can be >, <, = V shrinking

In practical applications, it is mostly the case that S3 is smaller than S2, which is different from ordinary hydraulic cylinders.

F push < F pull, V stretch > V pull

The purpose of doing this is to be different from the first hydraulic cylinder. This hydraulic cylinder is suitable for occasions requiring large pulling force and small thrust.

Differential hydraulic cylinder

The hydraulic cylinder of this structural form is an ordinary hydraulic cylinder, which is the first form. The difference is that the liquid supply method is different. When it is extended, the left chamber and the right chamber supply liquid at the same time, and when it is retracted, only the right chamber supplies liquid.

Differential structure

When stretching out, there is a pressure difference due to the different acting areas on both sides. Under the action of the thrust pressure difference, the rod moves to the right and stretches out, but the liquid discharged from the right chamber does not flow back to the liquid tank, but enters the left chamber together with the oil output from the hydraulic pump, thereby accelerating the stretching speed.

F push=PS1-PS2=PS3, when stretching out, the flow rate of the left chamber is the supply liquid flow rate Q+the flow rate of the right chamber QRight=VS2, so V=Q/S3, F pull=PS2, V contraction=Q/S2.

Compared with the first case, the pushing force has changed, but the pulling force has not changed.

Different results can be obtained by changing the area ratio of S1, S2, and S3.

This structure has been widely used in production, only need to add a differential valve.

Summarize

The above is a detailed introduction to the three basic working forms of double-acting hydraulic cylinders. Each working form of a double-acting hydraulic cylinder has its advantages and disadvantages. Therefore, we should pay special attention to this aspect when choosing a double-acting hydraulic cylinder.